Magnetically inductive sensors to determine a predetermined position of a translatory or rotatably movable machine element are well known; such positioning sensors are used in many applications, and one particular application is the derivation of signals to determine the ignition timing or ignition instant in an Otto motor by passing a gear, for example the starter gear, of an internal combustion engine adjacent a magnetically inductive pick-up coil. Gears, of course, transmit mechanical, rotary power. Starter gears of internal combustion engines are mechanically loaded only during short intervals of their rotation, namely only during starting of the associated engine. The gears induce alternating signals. One or more of the gear teeth should, however, provide a signal which differs from that of other signals so that a reference or marker signal can be obtained. Reference marker signals generating apparatus are also used in machine tools, for example to determine a limit position. For use with internal combustion engines, it is desirable to use an already existing gear, for example the starter gear, and to place a magnetic pick-up coil adjacent the starter gear. The reference marker, related to a particular angular position of the starter gear, is then utilized to control ignition timing of an internal combustion engine.
A pulse source wich has an inductive pick-up coil placed adjacent a machine component is described, for example, in German Patent Disclosure Document DE-OS No. 25 32 226 to which U.S. Pat. No. 4,121,112 corresponds. The teeth induce essentially uniform signals in the pick-up coils; the material of a predetermined tooth or of predetermined teeth differs from that of the material of other teeth, so that, when the predetermined tooth or teeth pass by the pick-up coil, the induced voltage which results will have a phase position which is shifted with respect to the phase position of the voltage induced upon passage of the other teeth by a predetermined minimum value.
It is difficult to manufacture such a positioning system since making the gears or treating the gears so that a predetermined tooth, or teeth, has different material is expensive. Substantial machining time and effort is necessary to place teeth of different material in a gear. If the pulse source is used, for example, in combination with the starter gear of an internal combustion (IC) engine, placing of a special tooth of different material on the gear is difficult since the teeth of the gear are subjected to substantial mechanical loading.